Methods and systems are known for monitoring the cardiac and respiratory activity of a patient and for generating an alarm in the event of cessation of breathing known as apnea, heartbeat cessation, or when the observed cardio-vascular activity surpasses a preset limit.
Such methods and systems find application in medical institutions for monitoring the condition of patients having respiratory or cardio-vascular diseases; monitoring newborn babies who may be susceptible to SIDS (Sudden Infant Death) syndrome, and in other clinical cases. As a rule, such monitoring is carried out by means of passive collection of information regarding a patient's condition, without attaching electrodes directly to the patient's body.
A method and system is known for round-the-clock monitoring of patients afflicted with OSAS (Obstructive Sleep Apnea) syndrome, which system comprises a special mattress with embedded sensors, electronic filters, a polygraph, etc. [see O. Polo, et al., "The Validity of the Static Charge Sensitive Bed in Detecting Obstructive Sleep Apneas," European Respiration Journal, Vol. 1, pp. 330-336 (1988)]. This method and system permit the recording of various movements of a sleeping patient, respiratory and cardiovascular deviations, and the like.
In U.S. Pat. No. 4,884,578, entitled "Apparatus for Monitoring Respiration and Cardiac Activity," there is disclosed an apparatus including a special bed mounted on a support suspended by cords or a universal joint and a device for restoring the bed to a normal position. Vertically and horizontally oriented sensors are embedded in the bed and connected to recording devices, permitting monitoring of a wide range of respiratory and cardiac activities.
In U.S. Pat. No. 4,381,788, entitled "Method and Apparatus for Detecting Apnea," there is described a monitor comprising a capacitance-type transducer pad, including two capacitor plates with resilient dielectric material between them and a special sensitive layer adapted to respond to the patient's respiratory motion. The pad is connected via an electric circuit to recording devices. A change of distance between the capacitor plates in response to the patient's movements causes a change of parameters in the sensor's electric circuit. The transduced signal is amplified, delayed for a certain, preset period of time and then, if preset parameters are exceeded, generates an alarm.
All of the above-mentioned methods and devices are complicated, expensive, require complicated instruments and can be applied only to patients confined to their beds and monitored under clinical conditions.
U.S. Pat. Nos. 4,657,025 and 4,738,264, entitled "A Heart and Breathing Alarm Monitor" disclose a monitor comprising a contactless sensor placed on a conventional bed away from a patient and having enhanced sensitivity in the vertical direction, a measuring transducer with electronic filters, and a recording device. The sensor detects the vertical vibrations of the bed produced by the patient's heartbeat and breathing functions, converts the detected vibrations into an electric signal and transmits the latter to the recording device. The device compares the obtained signal with preset circuits and, in the event the signals exceed a preset value, sets off an alarm. This monitor, however, can only be used on a patient confined to a bed and does not provide sufficient noise filtration. Moreover, its functional possibilities are rather limited. In particular, it cannot measure the rate of pulse wave propagation, which reflects the dynamics of the average arterial pressure.
It is an object of the present invention to provide general-purpose methods and devices for monitoring the physiological condition, not only of ill, but also of healthy, persons in clinical, ambulatory, home and working environments, which methods do not limit a person's activity in performing a job of average intensity in a sitting position. On the basis of such monitoring, the dynamics of the person's physiological condition and the necessity for its regulation or medical examination could be recognized.
It is a further object of the present invention to provide a method for measuring the rate of pulse wave propagation, correlated with the dynamics of a patient's average arterial pressure.
A still further object of the present invention is to provide a small, highly sensitive device for monitoring a patient's physiological condition which provides a more satisfactory filtration of noise, and which is of moderate cost.